Semiconductor diode lasers have found applications in a wide variety of information handling systems because of their compact size and because their technology is compatible with that of the associated electronic circuitry. These lasers are being used in areas such as data communication, optical storage and laser-beam printing and have been optimized for the particular applications with regard to their wavelength, optical power, their high speed modulation capabilities, beam quality, etc. For all applications, however, high performance devices require low threshold current values, i.e., the minimum current that needs to flow through the p-n junction of the laser diode to cause the coherent emission of light, should be as low as possible. This, in turn, results in a reduction of heat development and the associated cooling problems, an important factor particularly in high density packaging applications. Quantum well structures, in general, are well suited since they require lower currents than conventional heterostructures. An additional measure, resulting in a current reduction, is to provide structures with effective current confinement thereby avoiding unnecessary by-pass or stray currents.
Recently, very high performance low threshold current stripe lasers have been proposed where the active region of the laser is embedded within higher bandgap material in order to provide additional lateral carrier and optical confinement.
One such laser structure has been disclosed by P. L. Derry, et al. in an article "Ultralow-threshold graded-index separate-confinement single quantum well buried heterostructure (Al,Ga)As lasers with high reflectivity coatings," (Appl. Phys. Lett. 50 (25), Jun. 22, 1987, pp. 1773-1775).
Described in this article is a buried, graded-index separate-confinement heterostructure (GRINSCH) single quantum well (SQW) laser having a threshold current of below 1 mA. In this laser structure, high bandgap AlGaAs surrounds the active GaAs region in order to prevent carriers from diffusing out of the active region and to also provide for current confinement. The required fabrication process is rather complex involving (1) growing of the laser layers on a flat-surface substrate using molecular beam epitaxy (MBE) techniques, (2) etching grooves to form a ridge-patterned surface structure, and (3) performing a difficult and critical AlGaAs regrowth step using LPE techniques.
A different way of fabricating laser structures with effectively embedded active quantum wells (QW) has been described by E. Kapon. et al. in an article "Patterned quantum well semiconductor injection laser grown by molecular beam epitaxy," (Appl. Phys. Lett. 52 (8), Feb. 22, 1988, pp. 607-609).
Disclosed is an otherwise conventional GaAs/AlGaAs quantum well laser heterostructure that is MBE-grown on an (100)-oriented grooved substrate. Use is made of the fact that lateral variations in the thickness of the quantum wells occur when they are grown on a grooved surface. Due to the thickness variations, lateral patterning of the energy bandgap and the index of refraction is obtained.
Because of the groove geometries, the growth rate normal to the (111) or (411) planes of the groove sidewalls is remarkably lower than the growth rate on the (100)-plane. Injected carriers are confined to a narrow stripe, about 1 .mu.m wide, at the bottom of the groove because of the larger effective bandgap of the thinner QW layers on both sides of the stripe. Current confinement, desired to prevent the applied injection current from by-passing the p-n junction of the active region, is achieved by proton implantation whereby the walls of the groove are rendered non-conducting, leaving a conductive stripe, about 2 .mu.m wide, at the bottom of the groove. This additional process step is not self-aligned and thus limits further reduction in device size.